获取每组的前 1 行

Get top 1 row of each group(获取每组的前 1 行)

问题描述

我有一张表，我想获取每个组的最新条目.这是表:

I have a table which I want to get the latest entry for each group. Here's the table:

DocumentStatusLogs 表格

|ID| DocumentID | Status | DateCreated |
| 2| 1          | S1     | 7/29/2011   |
| 3| 1          | S2     | 7/30/2011   |
| 6| 1          | S1     | 8/02/2011   |
| 1| 2          | S1     | 7/28/2011   |
| 4| 2          | S2     | 7/30/2011   |
| 5| 2          | S3     | 8/01/2011   |
| 6| 3          | S1     | 8/02/2011   |

表格将按DocumentID 分组，并按DateCreated 降序排序.对于每个 DocumentID，我想获得最新状态.

The table will be grouped by DocumentID and sorted by DateCreated in descending order. For each DocumentID, I want to get the latest status.

我的首选输出:

| DocumentID | Status | DateCreated |
| 1          | S1     | 8/02/2011   |
| 2          | S3     | 8/01/2011   |
| 3          | S1     | 8/02/2011   |

• 是否有任何聚合函数可以只从每个组中获取顶部?参见下面的伪代码 GetOnlyTheTop:

  SELECT
    DocumentID,
    GetOnlyTheTop(Status),
    GetOnlyTheTop(DateCreated)
  FROM DocumentStatusLogs
  GROUP BY DocumentID
  ORDER BY DateCreated DESC
  

• 如果没有这样的功能，有什么办法可以实现我想要的输出吗?

• If such function doesn't exist, is there any way I can achieve the output I want?

  有关详细信息，请参阅父表:

  Please see the parent table for more information:

  Current Documents 表格

  Current Documents Table

  | DocumentID | Title  | Content  | DateCreated |
  | 1          | TitleA | ...      | ...         |
  | 2          | TitleB | ...      | ...         |
  | 3          | TitleC | ...      | ...         |
  

  父表是否应该是这样，以便我可以轻松访问其状态?

  Should the parent table be like this so that I can easily access its status?

  | DocumentID | Title  | Content  | DateCreated | CurrentStatus |
  | 1          | TitleA | ...      | ...         | s1            |
  | 2          | TitleB | ...      | ...         | s3            |
  | 3          | TitleC | ...      | ...         | s1            |
  

  更新我刚刚学会了如何使用应用"，它可以更轻松地解决此类问题.

  UPDATE I just learned how to use "apply" which makes it easier to address such problems.

  推荐答案

  ;WITH cte AS
  (
     SELECT *,
           ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) AS rn
     FROM DocumentStatusLogs
  )
  SELECT *
  FROM cte
  WHERE rn = 1
  

  如果您希望每天有 2 个条目，那么这将任意选择一个.要获得一天的两个条目，请改用 DENSE_RANK

  If you expect 2 entries per day, then this will arbitrarily pick one. To get both entries for a day, use DENSE_RANK instead

  归一化与否，看你愿不愿意:

  As for normalised or not, it depends if you want to:

  • 在 2 个地方保持状态
  • 保留状态历史
  • ...

  就目前而言，您可以保留状态历史记录.如果您也想要父表中的最新状态(这是非规范化)，您需要一个触发器来维护父表中的状态".或删除此状态历史记录表.

  As it stands, you preserve status history. If you want latest status in the parent table too (which is denormalisation) you'd need a trigger to maintain "status" in the parent. or drop this status history table.

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